Electronic reactance wave filter



Och 1940- w. VAN B. ROBERTS I 2,216,521

ELECTRONIC REACTANCE WAVE FILTER Filed June' 1, 1938 INVENTOR WAL 75? V /5Z!0B5QTS ATTORNEY Patented Oct. 1, 1940 PATENT .QFFlCE ELECTRONIC BEACTANOE WAVE FILTER Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, accrporation of Delaware Application June 1, 1938, Serial No. 211,129

3 Claims.

My present invention relates to wave filters generally, but especially to filter sections whose frequency discrimination is determined partly by passive reactance elements and partly by the transccnductance of electronic devices.

In my co-pending application Serial No. 103,230, filed Sept. 30, 1936 which issued as U. S. Patent 2,184,400, December 26, 1939, there is disclosed a filter section comprising a pair of shunt reactance elements connected to a transconductance device represented by a rectangle marked Y. @111 what follows such a device will be called a two-way transconductance device, or TWT for short. The object of the present invention is to provide filter sections comprising TWT devices in combination with reactance networks, other than those disclosed in my aforesaid application, thereby to provide filters having various characteristics, especially filters having a uniform magnitude of iterative impedance over the entire transmitting range, controllable inherent gain, and a band width independently controllable without physical alteration of reactance elements.

For convenience, a description of the properties of a TWT device together with an illustrative construction of such a device will be given before going on to a description of the various filters included in the present invention.

In the accompanying drawing, Fig. 1 illustrates a possible construction of a TWT device, and

Figs. 2 to 11, inclusive, show various combinations of reactances associated with a TWT device in such a way as to produce a filter section.

Referring specifically to Fig. 1, reference numeral I represents a screen grid amplifier tube having its plate and grid circuits energized with suitable direct operating voltages by way of choke 3' and impedance 3 respectively and having'its screen circuit connected to a suitable positive potential as indicated by the plus sign adjacent the lead to screen 4. The grid I2 is connected to one end of resistor 3. The source l2 of grid energizing potential is preferably adjustable. The cathode of tube I is connected to a line Whose terminal 6 acts as one input terminal for the impedance inverting circuit, while its terminal 1 acts as one output terminal for the device. Condensers 8 are blocking condensers. The other alternating current input terminal is denoted by numeral 9, and the other alternating output terminal by numeral It]. So far there has been described merely a conventional screen grid amplifier tube with means for separating its alternating input and output from its direct current energizing circuits. In accordance with the invention, however, a second screen grid tube H, arranged to provide negative transconductance between its input electrode l and its output electrode I6, is connected with its alternating current input voltage derived from terminal III, while its alternating output is connected to terminal 9. ,Its plate and grid circuits are energized in similar fashion to the plate and grid circuits of tube 1. In order that a negative transconductanc'e, may be obtained from screen grid tube ll, its

screen M is maintained at a direct potential sufficiently higher than the direct potential of plate l5, so that due to secondary-emission from plate it the current flow to plate I6 is reversed in" dimore negative the potential of input electrode rection but has a magnitude which is less the I5. The potential of plate 16 is, furthermore, ad-

If the transconductance of tube l is represented 5 by G12 and the absolute magnitude of the transconductance of tube II is represented by Gm,

then a positive increment of potential impressed upon input terminal 9 causes an increment of 1 current G12 to flow into the plate of tube l, as indicated in direction by the arrow alongside the lead thereto. Similarly, a positive increment of potential at output terminal l0 causes a current to'flow from plate 16 of tube H out through the input circuit connected between terminals 6 and 9 as indicated by the arrow above terminal 9. Thus the apparent impedance between input terminals 6 and 9, that is the ratio of the voltage applied thereto to the current fiowing therebetween in response to said voltage, depends upon the load impedance connected between terminals 10 and 1 across Which voltage is developed by the plate current of tube l.

Assuming an impedance Z connected between terminals 10 and 1, it will be seen that the potential developed at terminal 10, per volt applied to terminal 9, is equal to ZG12. As a result of this potential being applied to input electrode 15 of tube H a current flows from terminal 6 up through the input circuit to terminal 9, and, thence, in the opposite direction to the arrow adjacent terminal 9, into the plate IS. The magnitude of this current is ZG12G2I. Hence the apparent input impedance between terminals 6 and 9 is The power gain occurring in passing through a TWT device is calculated as follows: voltage e impressed between terminals 6 and 9 produces a current eGmGziZ between these terminals, but at the same time causes a current 8612 to flow between terminals 1 and ID. The voltage'drop across Z is therefore eGmZ andthe power developed in impedance Z is GZGZIZZ. The ratio of this power to the input power 62G12G2IZ is 9 G21 Fig. 2 shows a filter section consisting of two series elements 2 symmetrically connected to the input and output terminals of a TWT. Each of these elements 2 is reactive in nature, but may be composed of several elements forming a network having two terminals and the letter 2 represents the impedance between these two terminals. The iterative impedance of this filter section is readily shown to be The ratio of output current, or voltage, to input current, or voltage, in a properly terminated section is for the frequency range or ranges throughout which 1 Z ellen g. cm

the frequency range and the gain may be ad.- justed independently of each other by suitable adjustments of the separate values of G12 and G21.

Fig. 3 shows a filter section employing both series and shunt reactance networks and having a current, or voltage, gain per section again given G21 The band or bands of frequencies for which the transmission through such a section terminated by its iterative impedance is uniform, are those frequencies for which the iterative impedance is real. The iterative impedance of this section is:

Fig. 4 shows a different combination of series and shunt reactance networks to which all the remarks in connection with Fig. 3 apply, except that the iterative impedance of Fig. 4 is:

Fig. 5 is a section having a still more complicated structure. Its iterative impedance is and its gain and transmission bands are determined as mentioned in connection with Fig. 4.

Fig. 6 is still another structure subject to the same remarks except that its characteristic impedance is:

Fig. 7 shows a simplification of Fig. 2, arrived at by omitting one of the series reactance networks. The current ratio is, as always, given by throughout the transmitting band which is the band. for which wlwt.

which has a constant magnitude throughout the transmitting band.

Fig. 8 has the same properties as Fig. '7 except that the iterative impedance of the section shown in Fig. 8 is Z Zfi 05%. But this also has the constant magnitude Guam over the transmitting band defined by The filter sections, Figs. 7 and 8, have the interesting property that when a large number are used in cascade or one or more are terminated with their iterative impedance, the input impedance is constant over the band of uniform transmission which is a property not found in filters of the ordinary type, being only approximated to a certain degree in filters known as mderived types.

Fig. 9 shows an unsymmetrical section consisting of a TWT and a shunt reactance network across its output terminals. This also has an iterative impedance which is of constant magnitude Fig. 10 is the reverse transmission case of Fig. 9 and has an iterative impedance, as follows:

which again reduces to a constant absolute magthroughout the transmitting band.

Finally, it may be noted that if the TWT is at all frequencies and its current or voltage ratio is at all frequencies. Thus a cascaded arrangement of simple TWT sections is similar to a simple resistance coupled amplifier, except that its input impedance depends upon the value of G12G21 so that its input impedance and its gain may be independently adjusted.

It will be noted that in each of the arrangements, Figs. 2 to 11, the phase angle or the magnitude of the iterative impedance, or both, is constant throughout the range of frequencies in which transmission is uniform.

What I claim is:

1. An electric wave filter section whose iterative impedance has a component which is constant over a uniform transmission range of frequenoies of said section, comprising a two way transconductance device having transconductances or" opposite signs in the two directions of transmission therethrough, means for adjusting at least one of said transconductances and a passive reactance network associated with only one of said transconductances thereby forming a sec tion whose iterative impedance is constant in magnitude over a range of frequencies.

2. In combination with input and output terminals, a device for providing between the input terminals an apparentimpedance proportionalto the reciprocal of a given impedance between the output terminals, said device comprising a two way transconductance network having transconductances of opposite signs in the two directions of transmission therethrough, said network having input terminals coupled to said first named input terminals, and an element having said given impedance coupling said first named output terminals and the output terminals of said network.

3. In combination with input and output terminals, a device for providing between the input terminals an apparent impedance proportional to the reciprocal of a given impedance between the output terminals, said device comprising a twoway transconductance network having transconductances of opposite signs in the two directions of transmission therethrough, said network having input terminals coupled tosaid first named input terminals, an element having said given impedance coupling said first named output terminals and the output terminals of said network, and means for controlling the gain of one of the said transconductances thereby to adjust the magnitude of said apparent impedance.

WALTER VAN B. ROBERTS. 

